FREETOWN, March 22 (Reuters) - An outbreak of haemorrhagic fever that has killed 29 people in Guinea may have spread across the border into neighbouring Sierra Leone, according to a World Health Organisation (WHO) document and a senior Sierra Leone health official.

Guinean health officials have registered 49 cases of infection in three southeastern towns and the capital Conakry since the outbreak was first reported on Feb. 9.

While the exact type of the fever, which is characterised by bleeding, has yet to be identified, a senior official in Guinea said on Friday preliminary tests had narrowed down the possibilities to Ebola or Marburg Haemorrhagic Fever.

WHO officials, however, suspect Lassa Fever may be behind the outbreak, cases of which have now also been reported in a border region in Sierra Leone, according to minutes of a March 18 teleconference seen by Reuters.

Sierra Leone's Chief Medical Officer Dr. Brima Kargbo said authorities were investigating the case of a 14-year-old boy who died in the town of Buedu in the eastern Kailahun District.

The boy had travelled to Guinea to attend the funeral of one of the outbreak's earlier victims.

Kargbo said a medical team had been sent to Buedu to test those who came into contact with the boy before his death.

International medical charity Medecins Sans Frontieres (MSF) announced on Saturday it was reinforcing its team in Guinea. It is also flying in 33 tonnes of medicines and equipment and is setting up isolation units in three towns.

"These structures are essential to prevent the spread of the disease, which is highly contagious," Dr. Esther Sterk, MSF's Tropical Medicine Adviser, said in a statement. "Specialised staff are providing care to patients showing signs of infection"

Ebola and Marburg are lethal diseases caused by similar viruses that are among the most virulent pathogens known to infect humans, the WHO says on its website.

Humans contract Lassa Fever, which is endemic in West Africa, from contact with food or household items contaminated with rodent faeces. The disease can then be transmitted from person to person. (Reporting by Umaru Fofana; Writing by Joe Bavier; Editing by Gareth Jones)

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